<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <style>
        .x{
            display: flex;
            width: 500px;
            height: 500px;
            background-color: aqua;
            margin: 10px;
        }
        .x .left{
            width: 100px;
            background-color: rebeccapurple;
        }
        .x .middle{
            flex-grow: 1;
            background-color: aqua;
        }
        .x .right{
            flex-grow: 1;
            background-color: blue;
        }
    </style>
        <p class="x">
            <span class="left">1</span>
            <span class="middle">2</span>
            <span class="right">3</span>
        </p>
</body>
</html>